The invention relates generally to phosphor blends for wavelength conversion, and specifically phosphor blends for the conversion of radiation emitted by a light source. More particularly, the invention relates to phosphor blends for use with the blue light emitting diodes (LEDs).
A phosphor is a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits radiation energy in another portion of the electromagnetic spectrum. One important class of phosphors includes crystalline inorganic compounds of very high chemical purity and of controlled composition to which small quantities of other elements (called “activators”) have been added to convert them into efficient fluorescent materials. With the right combination of activators and inorganic compounds, the color of the emission can be controlled. Most useful and well-known phosphors emit radiation (also referred to as light herein) in the visible portion of the electromagnetic spectrum in response to excitation by electromagnetic radiation outside the visible range. For example, the phosphors have been used in mercury vapor discharge lamps to convert the ultra-violet (UV) radiation emitted by the excited mercury to visible radiation. Further, the phosphors may be used in a light emitting diode (LED) to generate colored emissions that may generally not be obtained from the LED itself.
Light emitting diodes (LEDs) are semiconductor light emitters often used as a replacement for other light sources, such as incandescent lamps. They are particularly useful as display lights, warning lights and indicating lights or in other applications where a colored light is desired. The color of light produced by an LED is dependent on the type of the semiconductor material used in its manufacture. The colored LEDs are often used in toys, indicator lights and other devices.
The colored semiconductor light emitting devices, including light emitting diodes and lasers (both are generally referred to as LEDs herein), have been produced from Group III-V alloys such as gallium nitride (GaN). With reference to the GaN-based LEDs, light is generally emitted in the UV and/or blue range of the electromagnetic spectrum. Until quite recently, the LEDs have not been suitable for lighting uses where a bright white light is needed, due to the inherent color of the light produced by the LEDs.
Techniques have been developed for converting the light emitted from the LEDs to useful light for illumination purposes. In one technique, the LED is coated or covered with a phosphor layer. The phosphor absorbs radiation generated by the LED, and generates radiation of a different wavelength, for example, in the visible range of the spectrum.
A combination of LED generated light and phosphor generated light may be used to produce white light. The most popular white LEDs are based on blue emitting GaInN chips. The blue emitting LEDs are coated with a phosphor or a phosphor blend including red, green and blue emitting phosphors that converts some of the blue radiation to a complementary color, for example a yellow-green emission. The total of the light from the phosphor and the LED chip provides white light having a color point with corresponding color coordinates (x and y) and correlated color temperature (CCT), and its spectral distribution provides a color rendering capability, measured by the color rendering index (CRI).
These white LEDs typically produces white light with a CRI between about 70 and about 80 for a tunable CCT greater than about 4000K. While such white LEDs are suitable for some applications, it is desirable to produce white light with higher CRIs (greater than about 90) and lower CCT (less than 3000K) for many other applications.
Therefore, it would be desirable to provide new and improved phosphor blends that produce white light with high CRI and high lumen for low CCT.